One of the biggest gripes when it comes to fixed-pitch propellers is … getting the pitch right. That is, how much it advances each revolution it makes (let’s leave out the difference between real pitch-how much a propeller actually advances in water-, geometric pitch-how much it would advance if it were turning in a solid-and regress-which is the difference between the two pitches).
How to find the right pitch?
Finding the right pitch depends on the boat, the type of engine, transmission and hull, etc. Trivially a planing boat and a displacement boat will have different needs. Just as on the same boat if you change some of the parameters such as weight, temperature and density of water you should change the propeller or, if you can its pitch. Because it is difficult to test multiple propellers before choosing the most suitable one, engine manufacturers generally provide standard ones suitable for the uses that are statistically most likely for the type of boat we have.
How to tell if the propeller is the right one?
For example, if I have a light motorboat, does the engine rev up quickly and stay at full speed easily, but the speed does not increase in the expected way? This means that I may need a propeller with a larger pitch, that is, a propeller that needs more thrust than one with a smaller pitch, but on the other hand, each of its turns “screws” deeper, or rather, further into the water for the benefit of navigational efficiency.
Conversely, I have a heavier vehicle and the engine cannot cope with unloading all its power into the water, I might be better off with a smaller pitch propeller than the one mounted . In both cases, the wrong propeller (and we refer only to the pitch and not to the shape of the blade) has as a likely additional side effect increased motor consumption.
So if I find that the propeller has a pitch that needs to be changed how do I remedy it?
Generally, either it is replaced tout court, or an attempt is made to correct the pitch by bringing in a trained technician. In both cases, it is necessary to pull the boat out of the water, work on the propeller, and then put the boat back on the water.
How to change your propeller without winging the boat
However, there are those who have found a different system to change the propeller pitch while minimizing lost time and cost. It is the Australian company Veem that has developed the Interceptor system for use with the propellers it makes in Perth.
To change the pitch, simply replace the interceptor strips, which are shims made of high-density polymer that are placed in special grooves near the trailing edge of each blade. The principle is simple and works somewhat like interceptors that are installed on the transom to change the longitudinal trim of the boat instead of flaps.
Using a strip (equipped with specific color according to its height so that it is easy to understand how to modify the propeller) higher the pitch increases and vice versa using a lower strip the pitch decreases. Veem propellers are certainly not cheap at the time of purchase, but they are advantageous for always having the ability to adjust their pitch even as often as needed just by using a diver, thus not winging the boat, which undoubtedly improves fuel consumption and prolongs engine life.